Bimetal motor step drive



Dec. 24, 1968 J. J. LESTER 3,418,499 I BIMETAL MOTOR STEP DRIVE FiledDec. 2'7, 1965 2 Sheets-Sheet 1 \uvENToR J HN J. LESTER A'r'roanev Dec,24, 1 968 J. J. LESTER BIMETAL MOTOR STEP DRIVE 2 Sheets-Sheet FiledDec. 27, 1965 Hui-P hwam'rog JO J. Las-raR ATTORNEY BY z /z/ UnitedStates Patent 3,418,499 BlME-TAL MOTOR STEP DRIVE John Joseph Lester,Brookfield, Wis., assignor, by mesne assignments, to Controls Company ofAmerica, Melrose Park, Ill., a corporation of Delaware Filed Dec. 27,1965, Ser. No. 516,266 11 Claims. (Cl. 310-4) ABSTRACT OF THE DISCLOSUREA heat motor operated step drive is disclosed wherein a bimetal operateddrive pawl and a latch pawl step a toothed wheel in response toenergization of a heater associated with the bimetal. The drive pawlslides on the latch pawl during the drive step and the heater circuit isintermittently made through the drive and latch pawls as they arealternately engaged and disengaged, a contact button providing themedium of engagement between the latch and drive pawls. The latch pawlincludes a guide for maintaining relative alignment of the latch pawl,drive pawl and toothed wheel, and the drive pawl has an articulatedconnection with the bimetal with the drive pawl being biased intoengagement with the latch pawl and bimetal. In another aspect, the drivemetal is supported by an ambient temperature compensating bimetal.

This invention relates to drive mechanisms and, more particularly, tostep drives of the type used, for example, to control switch operationin timers or the like.

Various step drive arrangements have been proposed in the past and somehave utilized a heat motor incorporating a bimetal-heater arrangement asthe source of motive power. An example of a heat motor using abimetal-heater arrangement is found in Patent No. 2,487,- 154 of Tom C.Lloyd and assigned, by mesne assignments, to the assignee of thisapplication. This invention is specifically concerned with animprovement in heat motor drives of the type disclosed in theaforementioned patent. In that patent the bimetal drive rotates aratchet wheel in response to energization of a bimetal heater and uses acombination of a drive pawl and latch pawl, the drive pawl steps theratchet wheel and the latch pawl holds a previously taken step while thedrive pawl is repositioned for a subsequent step. The latch and drivepawls complete the circuit to the bimetal heater and are separated tode-energize the bimetal heater at the completion of a particular stepand are re-engaged to reenergize the heater for another step when thedrive pawl has been returned to its initial driving position.

A general object of this invention is to provide an improved heat motordrive of this type.

Another object of this invention is to provide a mounting arrangement ofthe latch and drive pawls which insures a low resistance electricalconnection therebetween; and, moreover, an arrangement which maintainsthis low resistance connection throughout relative driving movementbetween the drive and latch pawls and regardless of the temperatureambient the heat motor.

A further object of this invention is to provide an improved supportarrangement for the drive pawl which insures proper alignment of thedrive pawl with the driven member.

A still further object is to provide an improved connection between thedrive pawl and bimetal element.

A further object of this invention is to provide an arrangement whichachieves ambient temperature compensation in a heat motor of this type;and, more specifically, an arrangement which maintains a substantiallyuniform operational cycle regardless of the ambient temperature.

Other objects and advantages will be pointed out in,

or be apparent from, the specification and claims, as will obviousmodifications of the embodiments shown in the drawings, in which:

FIG. 1 is a plan view of a switch mechanism and a heat motor drivenactuator constructed in accordance with this invention;

FIGS. 2 and 3 tions of the drive ratchet wheels;

FIG. 4 is a section 44 of FIG. 1;

FIG. 5 is a plan view of an alternative embodiment illustrating the heatmotor including ambient temperature compensation;

FIG. 6 is a plan view of a portion of the embodiment of FIG. 5illustrating the relative position of the elements in an elevatedambient temperature condition; and

FIGS. 7 and 8 contain operational curves for a heat motor without andwith ambient temperature compensation.

With particular reference to the drawings, a heat motor constructed inaccordance with this invention is illustrated as including drive pawl10, latch pawl 12, bimetal 14 and resistance heater 16. The drive andlatch pawls cooperate to step ratchet wheel 18. Gear 20 rotates withratchet wheel 18 and engages and drives gear 22 which in turn drives cam24, the directions of rotation of the various members are illustrated bythe arrows. Cam 24 is a part of a conventional timer switch arrangement26 the construction and operation of which is well known and, therefore,a specific description will not be made in this application. Forpurposes of understanding this invention it is believed sufiicient topoint out that switch blades 28, 30 and 32 are resilient and attached toterminals 34, 36 and 38 fixed to base 39. Rigid block 40 holds outerblades 28 and 32 out of engagement and maintains a minimum spacingtherebetween. Movable switch blade 30 normally engages switch blade 28and is engageable with switch blade 32 depending upon the position ofcam 24, specifically cam lobe 24a, with respect to movable switch blade30. Thus, the circuit established by timer switch 26 is determined bythe rotation of cam 24 and the cams position with respect to switchblade 30.

The drive for cam 24 is determined by drive pawl 10 and bimetal 14.Structurally, bimetal 14 is supported from bracket 44 attached to base39. Bimetal 14 extends from the bracket and is free to warp about itsconnection to bracket 44. The free end 46 of the bimetal extends intoslot 48 in web 50 of the drive pawl and coil spring 52 is connectedbetween post 54 attached to base 39 and tab 56 connected to web 50.Spring 52 extends angularly downwardly and back toward the bimetal tobias the drive pawl into engagement with the bimetal so that the drivepawl follows bimetal movement, and also biases the drive pawl intoengagement with the latch pawl which provides support for the free endof the drive pawl, an advantageous operational arrangement as will be described hereinafter.

Bimetal end 46 is reduced to provide shoulders 46a and 46b which engagethe underside of web 50 at slot 48 and cooperate in establishing theconnection between the bimetal and drive pawl. This connection limitslongitudinal movement of the drive pawl on the bimetal in one directionwhile freely permitting it in an opposite direction. Limited clearanceis provided between end 46 and the slot walls and this, together withthe just-described restrained and free relative longitudinal movement inopposite directions provides an articulated joint which permits limitedrelative movement between the drive pawl and bimetal to insure properalignment of the drive pawl with the bimetal illustrate successiveoperational posiand latch pawls with respect to the view taken generallyalong lines and also with the teeth of the ratchet wheel. Spring 52biases the drive pawl in the proper direction (i.e. it takes up all lostmotion in the connection between the bimetal and drive pawl) and,together with the latch pawl, permits the use of this articulatedconnection while insuring the necessary engagement between the drivepawl and bimetal to insure proper transmission of driving motion fromthe bimetal to the drive pawl.

The low expansion side of the bimetal faces toward ratchet wheel 18 sothat when resistance heater 16 is energized the bimetal bows toward theratchet wheel moving end 10a of the drive pawl into engagement withtooth 18a of the ratchet wheel. This movement of the drive pawl rotatesthe ratchet wheel counterclockwise.

Latch pawl 12 is resilient and is connected to base 58 fixed on base 39so that the latch pawl is fixed and the ratchet wheel and drive pawlmove with respect thereto. More specifically, as the drive pawl rotatesratchet wheel 18 counterclockwise the drive pawl slides along the latchpawl and tooth 18a moves away from and tooth 18b out from under the freeend of the latch pawl. During this movement the end of the latch pawlbows upwardly (FIG. 2) and eventually latch pawl 12 is released fromtooth 18b and snaps away from the drive pawl into engagement with thenext tooth (FIG. 3) for latching engagement with tooth 18b to insurethat the heat rnotor holds the step taken as a result of drive pawlmovement.

At the end of the desired step, resistance heater 16 is de-energizedallowing bimetal 14 to cool and causing the bimetal to move away fromthe ratchet wheel and return to its normal position. Spring 52 causesdrive pawl 10 to follow the bimetal and the drive pawl moves away fromtooth 18a. When the drive pawl clears tooth 1812, it is snapped backinto engagement with latch pawl 12 and is positioned for drivingengagement with tooth 18b whereupon the heat motor is ready for anotherstep.

Preferably the electrical circuit for resistance heater 16 isestablished through the latch and drive pawls so that the circuit ismaintained when the pawls are in contact during a drive step and it isautomatically interrupted when the latch and drive pawls separate at thecompletion of a particular step. More specifically, latch pawl base 58includes a terminal portion 60 to which an electrical lead (not shown)is connected and latch pawl 12 is made of electrically conductivematerial and is electrically and mechanically connected to terminalportion 60 by rivet 62. Preferably contact button 64 is engaged betweenthe opposed surfaces 63 and 65 of the latch and drive pawls and iscarried on the drive pawl. The contact button is spaced inwardly fromthe end of the drive pawl and when the bimetal is cold and it and thedrive pawl are in the relaxed position the contact button engages thelatch pawl at a point spaced from tooth 18b. With this arrangement,namely the latch pawl supporting the free end of the drive pawl, anadequate sliding surface is provided for the contact button and adequatecontact pressure between the contact button and the latch pawl ismaintained throughout the drive step. Spring 52 biasing the drive pawltoward the latch pawl maintains a low resistance connection between thecontact button and the latch pawl. The contact button and is particulararrangement with respect to the latch and drive pawls also permits thelatch pawl to bend around the contact button as the ratchet wheelrotates without pinching the latch pawl between the ratchet tooth andthe drive pawl and without disturbing the driven movement of the drivepawl.

The electrical connection to the resistance heater is completed throughthe drive pawl, which is also made of electrically conductive material,lead wire 66 extending between the drive pawl and terminal 68 on theresistance heater, with the opposite terminal 19 of the resistanceheater being connected to terminal 36 of the switch.

A further advantage in the illustrated embodiment resides in thespecific construction of drive pawl 10 and base 58. Base 58 includesguide portion 72 of electrical insulating material and having agenerally U-shapedconfiguration formed by base 74 and arms 76 and 78extending from the base toward the drive pawl. The drive pawl fits intothe open end of the guide and also includes arms 88 and 82 extendingfrom web 58 generally parallel to arms 76 and 78 of the guide. Guidearms 76 and 78 are aligned with the ratchet wheel and this arrangementguides the drive pawl in its driven movement to insure engagementbetween it and the ratchet wheel. Clearance is provided between the armsof the guide and drive pawl to permit limited movement of the drive pawlwith respect to the guide and the latch pawl to prevent stressing theelements but this clearance is limited so that movement is insufiicientto disengage either the drive pawl from tooth 18a or contact button 64from the latch pawl. Also it will be noted that the drive pawl is spacedfrom base 74 of the guide when the elements are in their relaxedposition and throughout driving movement to thereby insure maintainingadequate electrical engagement between the contact button and latch.

Starting with the bimetal in its normal, relaxed position (its coolposition) as illustrated in FIG. 1, energization of resistance heater 16through a suitable external source (not shown) will heat the bimetal andbow it toward ratchet wheel 18. Drive pawl 10 moves into engagement withratchet tooth 18a with guide 58 guiding drive pawl movement and button64 sliding along latch pawl 12 to maintain a low resistance connectionin the energizing circuit of the resistance heater. The drive pawlengages tooth 18a and rotates the ratchet wheel with tooth 18b bowingthe latch pawl toward the drive pawl. The arrangement of the buttonpermits this bowing Without pinching and unduly stressing or disturbingthe latch and drive pawls and insures an optimum electrical connectionduring relative movement between the drive and latch pawls. Spring 52also cooperates with the button in maintaining the low resistance jointeven though the latch pawl bows with respect to the drive pawl.Continued drive pawl movement and rotation of the ratchet wheeleventually moves tooth 18b from below the latch pawl and the latch pawl,by virtue of the internal biasing force created therein as it is bowedby tooth 18b, snaps away from the drive pawl. The latch pawl and buttonare separated interrupting the circuit to the electrical resistanceheater at the same time that the latch pawl is positioned for engagementwith tooth 18b to prevent the ratchet wheel backing up. At this pointthe free end of the drive pawl is supported on tooth 1811. Uponinterruption of the circuit to the heater the bimetal cools and startsto return to its normal relaxed position drawing the drive pawl awayfrom tooth 18a. Eventually the drive pawl will clear tooth 18b andspring 52 will snap the drive pawl into engagement with the latch pawl.The heater is re-energized and the bimetal bows to start another step ofthe heater motor in the above described manner.

The illustrated articulated joint between the bimetal and the drive pawlpermits movement of'the drive pawl for proper'driving engagement withthe ratchet wheel.

Throughout operation of the heat motor spring 52 biases the drive pawlin the proper sense with respect to the bimetal and the latch pawlthereby taking up all lost motion in the articulated joint andmaintaining a low resistance connection between the button and the latchpawl. Both of these features insure proper start-up char acteristicsregardless of the temperature ambient the heat motor, particularly withregard to the arrangement of spring 52 which insures a low resistancejoint regardless of the ambient temperature and the position of thedrive pawl and the bimetal as a result of that temperature.

In a heat motor drive of this type it is desirable to maintain the stepcycle relatively uniform regardless of the ambient temperature in whichthe heat motor is working. That is, the period from initial energizationof the heater to start a step through interruption of the heater circuitat the end of one step to re-establishment of the heater circuit toinitiate a subsequent step should remain relatively uniform regardlessof ambient temperature. The construction discussed to this point insuresproper electrical connection regardless of bowing of bimetal 14 due toambient temperature changes and it is proposed to achieve optimumambient temperature compensation in the sense of maintaining uniformcycling time by adjusting the position of the drive pawl and bimetalwith respect to the ratchet wheel in accordance with changes in ambienttemperature.

With particular reference to FIGS. 5 and 6, this adjustment is achievedby connecting main drive bimetal 90 to compensating bimetal 92.Structurally the connection is made through thermal insulators 94 and 96and rivet 98 also insulated from both bimetals 90 and 92. Bimetal 92 isfixed at one end to bracket 100 and bimetal 90 is connected and moveswith the free end of the compensating bimetal as it bows about its fixedend due to changes in ambient temperature. The high expansion side ofthe compensating bimetal faces toward the ratchet wheel and bimetal 90(downwardly in FIG. 5

When the ambient temperature increases bimetal 92 bows upwardly drawingbimetal 90 and drive pawl away from the ratchet (FIG. 6). This requiresthat heater 16' operate at a higher temperature to complete a drive stepso that the heater and bimetal 90 work at a higher temperature when theambient temperature increases. This maintains a relatively uniformtemperature differential between the bimetal and the ambient and sinceit is this differential which determines the rate of bimetal coolingwhich in turn determines cycle time a relatively uniform cycle ismaintained. This is illustrated graphically in FIGS. 7 and 8. Curve Aillustrates a step cycle at generally expected ambient temperatures,e.g. 75 F. At point X the drive step is initiated, the bimetaltemperature raises to point Y where the step is completed and the heatercircuit is interrupted. The bimetal then cools to point Z whereupon theheater circuit is re-established for another step with T being theelapsed time. Curve B (FIG. 7) illustrates the same heat motor operatingin an elevated ambient temperature, e.g. 125 F., and without ambienttemperature compensation. The bimetal starts at point X (substantiallythe same as X) and the bimetal temperature rises to point Y where thestep is completed and the bimetal starts to cool. Point Y isapproximately at the same temperature as Y but cooling must occur in ahigher ambient temperature where the differential between bimetal andambient temperature is reduced and cooling occurs at a much slower rateto point Z and the elapsed time T is greater than T.

Curve C (FIG. 8) illustrates a heat motor operating in an elevatedambient temperature, eg 125 F., and with temperature compensation. Inthis instance the step is initiated at X a higher temperature than X orX and the bimetal temperature increases to Y a higher temperature thaneither Y or Y before the step is completed and the heater de-energized.The bimetal is then cooled but the temperature differential between Yand ambient is substantially the same as between Y and ambient so thatcooling to Z occurs at substantially the same rate as from Y to Z andthe elapsed time T from X to Z is approximately equal to T. In testsusing the above mentioned ambient temperatures it was noted that Y and Ywere approximately 400 F. and Y was approximately 450 F. therebyestablishing the same temperature differential of 325 at both theexpected ambient temperature of 75 and the elevated ambient It will beappreciated that the same adjustment takes place in a reduced ambient.When the ambient tempera ture falls below, for example, 75 F. bimetal 92bows toward the ratchet wheel moving bimetal 90 and the drive pawltoward the ratchet wheel shortening the drive 6 stroke necessary tocomplete a step. This then reduces the temperature needed at bimetal tocomplete the step giving a lower operating temperature in a lowerambient to maintain a relatively uniform temperature differential.

In the selection of bimetals 90 and 92, bimetal 92 is made of moresensitive material to increase its responsiveness to ambient temperaturevariations. On the other hand, bimetal 90 is made of more rigid, lesssensitive material so that it is virtually insensitive to changes inambient temperature while still being responsive to the resistanceheater, the variations in ambient temperature being relatively small incomparison to the change in temperature produced in bimetal 90 by theheater. In sulators 94 and 96 serve to thermally isolate bimetal 92 frombimetal 90 so that the compensating bimetal is not affected by thetemperature of bimetal 90 as it is heated. Thus, bimetals 90 and 92 eachrespond to their own temperature condition without the temperaturecondition to which one is intended to respond adversely affecting theoperation of the other.

The embodiment of FIG. 5 also illustrates an alternative arrangement formounting the spring which maintains engagement of drive pawl 10' withthe latch pawl 12' and bimetal 90. Here spring 52' extends between tab56 on the drive pawl and a seat directly on bimetal 90. This arrangementmaintains the proper engagement between the drive pawl and bimetal sothat the drive pawl follows bimetal movement but isolates the springforces from bimetal movement so that the bimetal, or bimetals, are freeto bow without regard to the spring. This spring arrangement could alsobe used in the embodiment of FIGS. 1-4.

Another change in the heat motor is that heater terminal 69' isconnected to terminal 36' by a flexible lead 91 to permit more freedomof movement for bimetal 90. Otherwise the heat motor and switch are thesame as illustrated in FIGS. 1-4. It will also be noted that theillustrated arrangement of drive and latch pawls lends itselfparticularly well to provision of ambient temperature compensation. Moreparticularly, the drive pawl being supported on the latch pawl and beingfreely movable with respect to the latch pawl without affecting theelectrical connection between the drive and latch pawls permits thedrive pawl to be moved to correct for ambient temperature variationswithout affecting either electrical or mechanical operation.

It will also be noted that the spacing between drive pawl 10' and web 74of the latch pawl base 58 permits free movement of the drive pawl inresponse to movement of bimetal 92.

Although this invention has been illustrated and described in connectionwith particular embodiments thereof, it will be apparent to thoseskilled in the art that various changes and modifications may be madetherein without departing from the spirit of the invention or from thescope of the appended claims.

I claim:

1. A heat motor for driving a member having a toothed periphery andcomprising, in combination,

bimetal means,

electrically energized heater means positioned operatively adjacent saidbimetal means,

means supporting said bimetal means for movement in response toenergization and de-energization of said heater means,

a drive pawl connected to and movable with said bimetal means andextending toward said toothed periphery for selective engagement withthe teeth thereof,

a latch pawl adjacent said drive pawl and held against movement towardsaid member with said drive pawl, said latch pawl including at least aportion thereof extending coextensively with a portion of said drivepawl adjacent said toothed periphery and engaging the teeth of saidperiphery,

contact means connected to the projecting from one of said drive andlatch pawls and having limited engagement with the other of said driveand latch pawls, said contact means engaged between said drive and latchpawls holding said drive and latch pawls in spaced relationship andproviding a sliding electrical connection between said drive and latchpawls and a point about which said latch pawl bows as a result of drivenmovement of said member.

means biasing said drive pawl toward said latch pawl,

and means completing the electrical circuit for said heater meansthrough said latch and drive pawls.

2. The heat motor of claim 1 wherein said support means for said bimetalmeans includes compensating bimetal means connected to said bimetalmeans and also including means supporting said compensating bimetalmeans for movement relative to said member in response to changes in thetemperature ambient said heat motor with said compensating bimetal meansarranged to move said bimetal means, in response to an increase inambient temperature, in one direction to increase the movement of saiddrive bimetal means necessary to complete a step of said member and, inresponse to a decrease in said ambient temperature, in an oppositedirection to decrease the movement of said drive bimetal means tocomplete a step of said member.

3. A heat motor for driving a member having a toothed periphery andcomprising, in combination,

bimetal means,

electrically energized heater means positioned operatively adjacent saidbimetal means,

means supporting said bimetal means for movement in response toenergization and de-energization of said heater means,

a drive pawl connected to and movable with said bimetal means andextending toward said toothed periphery for selective engagement withthe teeth thereof,

a latch pawl adjacent said drive pawl and held against movement towardsaid member with said drive pawl, said latch pawl including at least aportion thereof extending coextensively with a portion of said drivepawl adjacent said toothed periphery and engaging the teeth of saidperiphery,

contact means engaged between said drive and latch pawls holding saiddrive and latch pawls in spaced relationship, said contact meansproviding -a sliding electrical connection between said drive and latchpawls and a point about which said latch pawl bows as a result of drivenmovement of said member,

means biasing said drive pawl toward said latch pawl,

means completing the electrical circuit for said heater means throughsaid latch and drive pawls,

said drive pawl being an elongated member and said latch pawl being anelongated member extending toward said periphery from a fixed mountdisposed intermediate the ends of said drive pawl,

said drive and latch pawl having opposed faces between which saidsliding electrical connection is made,

and said drive pawl moving longitudinally with respect to said latchpawl with said bimetal means.

4. The heat motor of claim 3 wherein said latch pawl extends from -afixed mount spaced from said toothed periphery and including meansdefining a guide extending toward said drive pawl,

said drive pawl disposed within said guide,

and said guide defining a path for said drive pawl in the direction ofmovement thereof with respect to said periphery, said guide havinglimited clearance with said drive pawl to permit a limited amount ofrelative movement between said drive pawl and said guide suflicient tomaintain said drive pawl substantially aligned with said toothedperiphery and said latch pawl.

5. The heat motor of claim 3 wherein said contact means comprises acontact button engaged between said latch and drive pawls and holdingsaid latch and drive pawls in relative spaced relation.

6. The heat motor of claim 3 wherein said member includes a toothedperiphery defining successive latch and drive portions and wherein saiddrive pawl comprises a substantially rigid member extending from saidpoint of connection toward said periphery with the distance between saidpoint of connection and said periphery determining the amount ofmovement and correspondingly the temperature of said drive bimetal meansnecessary to move said member through an increment of movement, andincluding compensating bimetal means connected to said drive bimetalmeans and positioning said drive bimetal means with respect to saidperiphery,

and means supporting said compensating bimetal means for movementrelative to said member in response to changes in the temperatureambient said heat motor to determine the position of said point ofconnection with respect to said periphery, said support means supportingsaid bimetal means to move said drive bimetal means and said point ofconnection away from said member in response to an increase in ambienttemperature and toward said member in response to a decrease in ambienttemperature.

7. The heat motor of claim 6 wherein said compensating bimetal meansextends from the support means therefor and said drive bimetal means isconnected to said compensating bimetal means at a point spaced from saidsupport means,

and including means at the point of connection between said drive andcompensating bimetal means for thermally isolating one from the other.

8. A heat motor for driving a member including means therein definingsuccessive latch and drive portions spaced in stepped increments andcomprising, in combination,

drive bimetal means, electrically energized heater means arranged inheat transfer relation with said drive bimetal means, means supportingsaid drive bimetal means for movement relative to said member inresponse to energization and de-energization of said heater means, adrive pawl disposed operatively adjacent said mem- Iber,

a latch pawl disposed operatively adjacent said mem ber and a portion ofsaid latch pawl disposed adjacent a portion of said drive pawl,

connection means defining a connection between said drive pawl and drivebimetal means which prevents movement of said drive pawl in onedirection with respect to said drive bimetal means and permits movementof said drive pawl in another direction with respect to said drivebimetal means,

means biasing said drive pawl in said one direction with respect to saiddrive bimetal means so that said drive pawl is movable with and followssaid bimetal means and also biasing said drive pawl into engagement withsaid latch pawl so that said drive pawl is also supported by said latchpawl,

means providing a sliding electrical connection between said drive andlatch pawls,

and means completing an electrical circuit for said heater means throughsaid latch and drive pawls, energization of said drive bimetal meansbeing maintained through said sliding connection as said drive pawlmoves in response to bimetal means movement to drive said member, saidsliding connection being interrupted when said member has been moved asuflicient distance to engage said latch pawl in a successive latchposition and being re-established when said drive pawl is returned bysaid drive bimetal means to a successive drive position.

9. The heat motor of claim 8 wherein said member includes a toothedperiphery and said successive latch and drive portions are defined bythe teeth of said periphery,

said latch pawl extends from a fixed mount spaced from said toothedperiphery and including means defining a guide extending toward saiddrive pawl,

said drive pawl disposed within said guide,

and said guide defining a path for said drive pawl in the direction ofmovement thereof with respect to said periphery and aligned with saidtoothed periphery, said guide having limited clearance with said drivepawl to permit a limited amount of relative movement between said drivepawl and said guide sufiicient to maintain said drive pawl substantiallyaligned with said toothed periphery and said latch pawl.

10. The heat motor of claim 8, wherein said connection means comprises aslot in one and a complementary shouldered projection in the other ofsaid drive pawl and drive bimetal means,

said projection disposed in said slot and said biasing means biasing theshoulders of said projection and the walls of said slot into engagement.

11. The heat motor of claim 8 wherein said support means for saidbimetal means includes compensating bimetal means connected to saidbimetal means and also including References Cited UNITED STATES PATENTS952,778 3/1910 Wohl et a1. 3104 2,326,002 8/ 1-943 Baak 3104 X 2,355,0418/1944 Baak 310-4 2,487,154 11/1949 Lloyd 3104 2,707,737 5/ 1955 Rich etal 3104 X FOREIGN PATENTS 800,047 8/ 1950 Germany.

I. D. MILLER, Primary Examiner. D. F. DUGGAN, Assistant Examiner US. Cl.X.R. 58-23

